Single-photon Polarization Qubit Research Articles

Wavelength conversion for single-photon polarization qubits through continuous-variable quantum teleportation

A quantum internet connects remote quantum processors that need interact and exchange quantum signals over a long distance through photonic channels. However, these quantum nodes are usually composed of quantum systems with emitted photons unsuitable for long-distance transmission. Therefore, quantum wavelength conversion to telecom is crucial for long-distance quantum networks based on optical fiber. Here we propose wavelength conversion devices for single-photon polarization qubits using continuous variable quantum teleportation, which can efficiently convert qubits between near-infrared (780/795 nm suitable for interacting with atomic quantum nodes) and telecom wavelength (1300-1500 nm suitable for long-distance transmission). The teleportation uses entangled photon sources (i.e., non-degenerate two-mode squeezed state) that can be generated by four-wave mixing in rubidium atomic vapor cells, with a diamond configuration of atomic transitions. The entangled fields can be emitted in two orthogonal polarizations with locked relative phase, making them especially suitable for interfacing with single-photon polarization qubits. Our work paves the way for the realization of long-distance quantum networks.

Efficient quantum memory for photonic polarization qubits generated by cavity-enhanced spontaneous parametric downconversion.

Quantum memories, for storing then retrieving photonic quantum states on demand, are crucial components for scalable quantum technologies. Spontaneous parametric downconversion (SPDC) with a nonlinear crystal is the most widely used process for generating entangled photon pairs or heralded single photons. Despite the desirability of efficient quantum memories for SPDC-generated single photons, the storage and retrieval efficiencies achieved with this approach still fall below 50%, a threshold value for practical applications. Here, we report an efficiency of > 70% for the storage of heralded single photons generated by cavity-enhanced SPDC using atomic quantum memories based on electromagnetically induced transparency (EIT). In addition, we demonstrate the quantum memory for single-photon polarization qubits with a fidelity of ∼96%. This result paves the way towards the development of large-scale quantum networks.

Quantum memory of single-photon polarization qubits via double electromagnetically induced transparency

We present a theoretical investigation on the quantum memory of photonic polarization qubits via electromagnetically induced transparency (EIT). The system we consider is a tripod-shaped four-level atomic system working under the condition of a double EIT, by which the storage and retrieval of a single-photon polarization qubit are implemented through the switching off and on of a control laser field, and the storage efficiency and the quantum-state fidelity for qubit memory are both calculated. We show that the optimal optical depth for acquiring the maximum efficiency and maximum fidelity of the qubit memory can be obtained simultaneously, which can be further improved by suppressing the optical absorption and dispersion via the choice of the time duration of the input qubit pulse and the amplitude of the control field. We also carry out a calculation on the quantum memory of a single-photon qudit by considering a multipod-shaped atomic-level configuration. The results reported here are useful for understanding the quantum transmission property of slow lights with multiple components and helpful for experimental realizations of high-quality memory of photonic qubits and qudits.

Scalable Quantum Controlled Gates on Single-Photon Polarization Qubits Assisted by Nitrogen-Vacancy Centers Inside Single-Sided Optical Cavities

Scalable Quantum Controlled Gates on Single-Photon Polarization Qubits Assisted by Nitrogen-Vacancy Centers Inside Single-Sided Optical Cavities

Optical amplification of single-photon polarization qubit using weak measurement

Single-photon amplification can be used to overcome the photon loss in long-distance quantum communication. However, the trade-off of success probability and amplification gain should be further improved. We propose a single-photon polarization qubit amplification using weak measurement. The polarization qubit can be amplified by the pre- and post-selection of the single-photon meter state weakly coupling with the input state. The success probability does not decrease asymptotically to 0 with increasing amplification gain in the presented proposal, and is higher than that in entanglement-based schemes, without coincidence detection required. Our proposal is feasible in current technology, and helpful for near-future quantum communication.

Teleportation of a multiphoton qubit using hybrid entanglement with a loss-tolerant carrier qubit

It was shown that using multiphoton qubits, a nearly deterministic Bell-state measurement can be performed with linear optics and on-off photodetectors [Phys. Rev. Lett. 114, 113603 (2015)]. However, multiphoton qubits are generally more fragile than single-photon qubits under a lossy environment. In this paper, we propose and analyze a scheme to teleport multiphoton-qubit information using hybrid entanglement with a loss-tolerant carrier qubit. We consider three candidates for the carrier qubit: a coherent-state qubit, a single-photon polarization qubit, and a vacuum-and-single-photon qubit. We find that teleportation with the vacuum-and-single-photon qubit tolerates about 10 times greater photon losses than with the multiphoton qubit of the photon number $N \geq 4$ in the high fidelity regime ($F\geq 90\%$). The coherent-state qubit encoding may be even better than the vacuum-and-single-photon qubit as the carrier when its amplitude is as small as $\alpha<0.78$. We further point out that the fidelity of the teleported state by our scheme is determined by loss in the carrier qubit while the success probability depends on loss only in the multiphoton qubit to be teleported. Our study implies that the hybrid architecture may complement the weaknesses of each qubit encoding.

Efficient quantum memory for single-photon polarization qubits

A quantum memory, for storing and retrieving flying photonic quantum states, is a key interface for realizing long-distance quantum communication and large-scale quantum computation. While many experimental schemes of high storage-retrieval efficiency have been performed with weak coherent light pulses, all quantum memories for true single photons achieved so far have efficiencies far below 50%, a threshold value for practical applications. Here, we report the demonstration of a quantum memory for single-photon polarization qubits with an efficiency of >85% and a fidelity of >99 %, basing on balanced two-channel electromagnetically induced transparency in laser-cooled rubidium atoms. For the single-channel quantum memory, the optimized efficiency for storing and retrieving single-photon temporal waveforms can be as high as 90.6 %. Our result pushes the photonic quantum memory closer to its practical applications in quantum information processing.

Ultralow-Noise Room-Temperature Quantum Memory for Polarization Qubits

Here we show an ultra-low noise regime of operation in a simple quantum memory in warm Rb atomic vapor. By modelling the quantum dynamics of four-level room temperature atoms, we achieve fidelities >90% for single-photon level polarization qubits, clearly surpassing any classical strategy exploiting the non-unitary memory efficiency. This is the first time such important threshold has been crossed with a room temperature device. Additionally we also show novel experimental techniques capable of producing fidelities close to unity. Our results demonstrate the potential of simple, resource-moderate experimental room temperature quantum devices.

Generation of hybrid entanglement between a single-photon polarization qubit and a coherent state

We propose a scheme to generate entanglement between a single-photon qubit in the polarization basis and a coherent state of light. The required resources are a superposition of coherent states, a polarization entangled photon pair, beam splitters, the displacement operation, and four photodetectors. Even when realistic detectors with a limited efficiency are used, an arbitrarily high fidelity can be obtained by adjusting a beam-splitter ratio and the displacement amplitude at the price of reducing the success probability. Our analysis shows that high fidelities may be obtained using on-off detectors with low efficiencies and available resource states under current technology.

Polarization properties of long-lived stimulated photon echo

The polarization properties of the long-lived stimulated photon echo formed on the transition ja → jb with the atomic levels degenerate in the projections of the angular momenta are studied theoretically. The two particular transitions ja = 1 → jb = 0 and ja = 1 → jb = 1 with degenerate ground state ja = 1 are discussed. For the transitions ja = 1 → jb = 1 the polarizations and areas of the first (‘write’) and the third (‘read’) excitation pulses are found when the echo polarization faithfully reproduces the arbitrary polarization of the weak (single-photon) second (‘information’) pulse, so that this echo scheme may implement the quantum memory for a single-photon polarization qubit, while for the transitions ja = 1 → jb = 0 it is shown, that the echo polarization differs from that of the second pulse at any conditions.

Protecting a quantum memory for a photonic polarization qubit in a cold atomic ensemble by dynamical decoupling.

We report an experimental demonstration of storage of photonic polarization qubit (PPQ) protected by dynamical decoupling (DD). PPQ's states are stored as a superposition of two spin waves by electromagnetically-induced-transparency (EIT). Carr-Purcell-Meiboom-Gill (CPMG) DD sequences are applied to the spin-wave superposition to suppress its decoherence. Thus, the quantum process fidelity remains better than 0.8 for up to 800 μs storage time, which is 3.4-times longer than the corresponding storage time of ~180 μs without the CPMG sequences. This work is a key step towards the storage of single-photon polarization qubit protected by the CPMG sequences.

Experimental violation and reformulation of the Heisenberg's error-disturbance uncertainty relation

The uncertainty principle formulated by Heisenberg in 1927 describes a trade-off between the error of a measurement of one observable and the disturbance caused on another complementary observable such that their product should be no less than the limit set by Planck's constant. However, Ozawa in 1988 showed a model of position measurement that breaks Heisenberg's relation and in 2003 revealed an alternative relation for error and disturbance to be proven universally valid. Here, we report an experimental test of Ozawa's relation for a single-photon polarization qubit, exploiting a more general class of quantum measurements than the class of projective measurements. The test is carried out by linear optical devices and realizes an indirect measurement model that breaks Heisenberg's relation throughout the range of our experimental parameter and yet validates Ozawa's relation.

Quantum process tomography of single-photon quantum channels with controllable decoherence

We characterized unital quantum channels of single-photon polarization qubits. The channels are composed of two birefringent crystals and wave plates, where their decoherence properties are controlled. An experimental comparison of two different depolarizing configurations was made using a quantum process tomography procedure. The results are with a high fidelity to theoretical predictions.

Minimum-disturbance measurement without postselection

We propose and demonstrate a linear optical device which deterministically performs optimal quantum measurement or minimum disturbance measurement on a single-photon polarization qubit with the help of an ancillary path qubit introduced to the same photon. We show theoretically and experimentally that this device satisfies the minimum disturbance measurement condition by investigating the relation between the information gain (estimation fidelity) and the state disturbance due to measurement (operation fidelity). Our implementation of minimum disturbance measurement is postselection-free in the sense that all detection events are counted toward evaluation of the estimation fidelity and the operation fidelity, i.e., there is no need for coincidence postselection of the detection events.